This invention relates to solid fuel ramjet engines and more particularly to a device for controlling the generation of fuel rich combustion gases from the solid fuel. In still greater particularity the invention relates to a device for controlling the generation of fuel by changing the distribution of air within the solid fuel combustor in response to changes in engine and flight conditions encountered in operation.
Solid fuel ramjet engines, whether brought to operational speed by a booster engine or air dropped from a vehicle, depend upon the introduction of air into the engine due to its forward motion. Thus the term ramjet is used. As the ram air passes through a solid fuel grain within a combustor, fuel rich gases generated by the solid fuel react with oxygen in the air in a combustion chamber aft of the combustor and pass out of the engine via a nozzle producing thrust.
The rate at which the fuel rich gases are generated from the solid fuel grain, in general, depends on the fuel grain surface area, temperature and pressure within the combustor and the mass rate of air flow over the fuel grain. Due to wide and uncontrollable flight conditions encountered by the engine during operation, the air mass flow varies considerably and not necessarily in a predictable manner. Without some means of controlling the burn rate of the solid fuel in response to changes in air mass flow excessively rich combustion chamber conditions will exist, which is very wasteful of fuel and reduces the range of the vehicle. Additionally, engine variables, such as changes in the solid fuel grain area, thrust, and combustor temperatures and pressures, as well as missile flight parameters, such as Mach number and angle of attack necessitate changes in fuel burn rate to maintain the variable within acceptable limits.
Heretofore, efforts have been directed to engine configurations that either bypass a portion of the ram air around the solid fuel combustor or vent a portion of the air outside of the vehicle. In both cases the object is to reduce the air flow into the solid fuel combustor and reduce the burn rate of the fuel.
Another common technique used to control the uniformity, but not directly the rate of fuel burning, includes a tube-in-hole arrangement, whereby a tube is inserted into the fuel grain inlet. The tube splits the air flow and improves the uniformity of burning of the solid fuel grain. Typical arrangements are disclosed in U.S. Pat. Nos. 4,031,691 and 4,052,846.
A device for controlling the rate of fuel burning, in the combustor, but not necessarily the uniformity thereof, is disclosed in U.S. Pat. No. 3,844,118, wherein a valve moves to restrict the air inlet to the fuel grain in response to engine conditions, rather than ambient air conditions, to vary the total mass flow of air into the solid fuel combustor.
The tube-in-hole technique has proven effective in improving the distribution of air within the solid fuel combustor and uniformity of the fuel burn; however, these devices do not provide for continuously changing the distribution of air within the combustor in response to instantaneous changes in mass air flow or engine and missile parameters encountered in flight. Rather, the tube-in-hole is designed to give more uniform burning for a nominal flight condition, and over rich fuel conditions still present a problem.
The valve arrangement set out, and other devices for throttling the air into the combustor, while effective in changing the total air flow through the combustor in a predetermined manner, and, thereby achieving a change in fuel burn rate, do not achieve the change in fuel burn rate by controlling the distribution of air over the fuel grain surface in response to instantaneous changes in air mass flow encountered. Rather, the valve arrangement responds only to engine variables, such as increasing fuel grain area or thrust, to throttle the air flow into the combustor, causing fuel production to increase or decrease, thereby achieving the desired result.